A pneumatic rock drill assembly normally includes a rotary casing, a driver sub having a threaded connection with the casing, and a drill bit having a splined connection with the driver sub. The splined connection between the driver sub and the drill bit enables the drill bit to rotate with the driver sub and to move axially relative to the driver sub and the casing as the drill bit is driven into the rock or other strata being drilled. Compressed air is used to drive a piston in a reciprocating motion, and during each downstroke the piston strikes the drill bit to drive the drill bit into the rock or other strata being drilled.
A retaining ring retains (secures) the drill bit in the drill assembly. The retaining ring is disposed to engage a retaining shoulder on the drill bit when the drill bit has been driven a certain distance relative to the casing (to a position known as the "drop open" position). The retaining ring resists additional movement of the drill bit, thereby retaining the drill bit in the drill assembly.
One conventional type of retaining ring is an annular split ring with an elastomeric band about its outer periphery. The retaining ring is relatively massive and stiff and has not been designed to absorb shock. The elastomeric band facilitates assembly and alignment of the annular retaining ring in the drill casing, but has no appreciable effect on the ring's shock absorbing capabilities.
Under normal drilling conditions, the resistance of the rock being drilled takes up most of the impact forces generated during drilling. The piston drives the drill bit a short distance into the rock ahead of it, but the drill bit is normally not driven to its "drop open" position. However, if poor drilling practices are followed, or if the rock is exceptionally weak or unconsolidated, the impact of the piston can drive the drill bit to its "drop open" position. When that happens, the retaining shoulder on the drill bit impacts the retaining ring. With a retaining ring that is massive and stiff, impact forces are either reflected back to the drill bit or transmitted to the threaded connection between the driver sub and the casing. Such impact forces can cause deformation of the retaining shoulder of the drill bit and can contribute to premature fatigue failure in the threaded connection between the casing and the driver sub.
In percussion drill assemblies, the use of spring shock absorbers in addition to conventional retaining rings is known. For example, an Ingersoll-Rand percussion drill assembly uses a conventional annular split retaining ring, and an additional spring structure to absorb shocks. The additional spring structure comprises a stack of four annular Belleville springs disposed between a piston bumper and the casing. As the drill bit is driven toward its "drop open" position, the retaining shoulder of the drill bit passes through the annular Belleville springs and engages the split retaining ring in the usual manner. At the same time, the piston contacts the piston bumper, which engages the Belleville springs to absorb some measure of the impact load. The springs used in the Ingersoll-Rand drill are conventional Belleville springs (i.e., complete rings).